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Iraq aims to retake key city of Falluja from ISIS after months of siege

Iranian-trained forces are likely to play big role; ISIS has had plenty of time to dig in

(CNN)Iraqi forces are "approaching a moment of great victory," Prime Minister Haider al-Abadi declared Sunday night on television, flanked by senior military and police commanders. The long-anticipated offensive to drive ISIS out of the city of Falluja has begun.

But how long it will take to reach that moment is difficult to say. ISIS has proven to be a formidable foe, and every victory over the group has come at a high cost. To paraphrase that classic line attributed by war correspondent Peter Arnett to an American officer in the Vietnam War: To save cities from ISIS, Iraqi forces have had to destroy them.

The Iraqi air force has dropped thousands of leaflets over Falluja urging civilians to leave, but ISIS is reportedly preventing them from doing so. The leaflets advise residents to raise white flags over their homes if they can't flee. That there will be innocent civilian victims, and many of them, is not a possibility: It's a certainty.

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Falluja has been under siege by Iraqi forces for months. The humanitarian situation is dire, with severe shortages of food and medicine. The Iraqi military, bolstered by recently delivered U.S.-made F-16 fighter aircraft, has been bombarding targets in and around the city for weeks.

Falluja is a traditionally Sunni-dominated city about 65 kilometers (40 miles) west of Baghdad, once with a population of 300,000, now down to less than a third of that. Falluja was the scene of some of the bloodiest fighting between U.S. forces and insurgents during the American occupation of Iraq, and it became the first Iraqi city ISIS captured in January 2014.

Iran-trained Shia fighters

The forces participating in the offensive are a mix of Iraqi army, police, anti-terrorism units, Sunni tribal militias and what are known as the Popular Mobilization Units, or PMU -- Shia-dominated militias well-armed and trained by Iran. U.S. warplanes will provide some air cover but will have to do so selectively since the United States is eager to avoid the impression it's providing air support for pro-Iranian forces.

The U.S.-led anti-ISIS coalition said it has carried out 21 air and drone strikes on ISIS targets in Falluja since Tuesday.

This hodgepodge of fighting units have been in action before -- during the offensives to retake Tikrit and Baiji last year. The PMU seemed better organized, equipped and led. I attended a briefing chaired by senior PMU commander Abu Mahdi al-Muhandis outside Baiji in June. The senior Iraqi army generals at the briefing were highly deferential, clearly the junior partners in the operation.

Sunni fighters, plus select government forces, will in theory take the lead when it comes to fighting inside the city to avoid the impression Shia-dominated forces are conquering and occupying Falluja, a proudly Arab Sunni city.

I met some of those Sunni fighters last summer when they graduated from basic training. They complained of not getting paid, of having to buy their own uniforms, of being begrudgingly provided with poor weapons and inadequate ammunition. Morale was clearly low, distrust of the Shia-dominated government in Baghdad high.

Unrest in the capital

The Prime Minister vowed Iraq will "tear down the black banners of the strangers who kidnapped" the city of Falluja. The buildup to the operation has been covered extensively in the Iraqi media, with operational secrecy sacrificed to the need to bolster a beleaguered government's credentials.

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While Iraqi forces have been successful at retaking territory from ISIS, including the cities of Tikrit, Baiji, Ramadi and, last week, the town of Rutba on the highway to Jordan, the throne in Baghdad is shaky.

The Falluja offensive takes place against a background of mounting political turmoil in Baghdad, where twice in the last three weeks protesters loyal to Shiite cleric Muqtada al-Sadr have marched on the so-called Green Zone, the heart of the government.

They occupied the Iraqi parliament and the Prime Minister's office, demanding an end to government corruption, repeating a chant oft-heard during the Arab Spring: "The people want to topple the regime." At least two protesters were killed over the weekend in the latest Green Zone occupation.

Bigger challenge ahead

People flee the scene of a terror attack at Istanbul's Ataturk airport on June 29. Turkish officials have strong evidence that ISIS leadership was involved in the planning of the attack, a senior government source told CNN. Officials believe the men -- identified by state media as being from Russia, Uzbekistan and Kyrgyzstan -- entered Turkey from the ISIS stronghold of Raqqa in Syria, bringing with them the suicide vests and bombs used in the attack, the source said.

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Photos:The ISIS terror threat

The ISIS militant group -- led by Abu Bakr al-Baghdadi, pictured -- began as a splinter group of al Qaeda. Its aim is to create an Islamic state, or caliphate, across Iraq and Syria. It is implementing Sharia law, rooted in eighth-century Islam, to establish a society that mirrors the region's ancient past. It is known for killing dozens of people at a time and carrying out public executions.

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Photos:The ISIS terror threat

Kurdish Peshmerga fighters fire missiles during clashes with ISIS in Jalawla, Iraq, on June 14, 2014. That month, ISIS took control of Mosul and Tikrit, two major cities in northern Iraq.

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Traffic from Mosul lines up at a checkpoint in Kalak, Iraq, on June 14, 2014. Thousands of people fled Mosul after it was overrun by ISIS.

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ISIS fighters parade down an Iraqi street in this image released by the group in July 2014.

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Photos:The ISIS terror threat

Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and other Yazidi people are flown to safety after a dramatic rescue operation at Iraq's Mount Sinjar on August 11, 2014. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. Only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

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Photos:The ISIS terror threat

On August 19, 2014, American journalist James Foley was decapitated by ISIS militants in a video posted on YouTube. A month later, they released videos showing the executions of American journalist Steven Sotloff and British aid worker David Haines.

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Photos:The ISIS terror threat

ISIS militants stand near the site of an airstrike near the Turkey-Syria border on October 23, 2014. The United States and several Arab nations began bombing ISIS targets in Syria to take out the group's ability to command, train and resupply its fighters.

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A Kurdish marksman stands atop a building as he looks at the destroyed Syrian town of Kobani on January 30, 2015. After four months of fighting, Peshmerga forces liberated the city from the grip of ISIS.

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Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on February 4, 2015. Al-Kasasbeh's son, Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.

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Photos:The ISIS terror threat

In February 2015, British newspapers report the identity of "Jihadi John," the disguised man with a British accent who had appeared in ISIS videos executing Western hostages. The militant was identified as Mohammed Emwazi, a Kuwaiti-born Londoner. On November 12, 2015, the Pentagon announced that Emwazi was in a vehicle hit by a drone strike. ISIS later confirmed his death.

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Photos:The ISIS terror threat

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Photos:The ISIS terror threat

In March 2015, ISIS released video and images of a man being thrown off a rooftop in Raqqa, Syria. In the last photograph, the man is seen face down, surrounded by a small crowd of men carrying weapons and rocks. The caption reads "stoned to death." The victim was brutally killed because he was accused of being gay.

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Photos:The ISIS terror threat

An Iraqi soldier searches for ISIS fighters in Tikrit on March 30, 2015. Iraqi forces retook the city after it had been in ISIS control since June 2014.

Dead bodies lie near a beachside hotel in Sousse, Tunisia, after a gunman opened fire on June 26, 2015. ISIS claimed responsibility for the attack, which killed at least 38 people and wounded at least 36 others, many of them Western tourists. Two U.S. officials said they believed the attack might have been inspired by ISIS but not directed by it.

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Photos:The ISIS terror threat

ISIS also claimed responsibility for what it called a suicide bombing at the Al-Sadiq mosque in Kuwait City on June 26, 2015. At least 27 people were killed and at least 227 were wounded, state media reported at the time. The bombing came on the same day as the attack on the Tunisian beach.

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A man inspects the aftermath of a car bombing in Khan Bani Saad, Iraq, on July 18, 2015. A suicide bomber with an ice truck, promising cheap relief from the scorching summer heat, lured more than 100 people to their deaths. ISIS claimed responsibility on Twitter.

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Two women hold hands after an explosion in Suruc, Turkey, on July 20, 2015. The blast occurred at the Amara Cultural Park, where a group was calling for help to rebuild the Syrian city of Kobani, CNN Turk reported. At least 32 people were killed and at least 100 were wounded in the bombing. Turkish authorities said they believed ISIS was involved in the explosion.

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Spectators at the Stade de France in Paris run onto the soccer field after explosions were heard outside the stadium on November 13, 2015. Three teams of gun-wielding ISIS militants hit six locations around the city, killing at least 129 people and wounding hundreds.

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Law enforcement officers search a residential area in San Bernardino, California, after a mass shooting killed at least 14 people and injured 21 on December 2, 2015. The shooters -- Syed Rizwan Farook and his wife, Tashfeen Malik -- were fatally shot in a gunbattle with police hours after the initial incident. The couple supported ISIS and had been planning the attack for some time, investigators said.

Two wounded women sit in the airport in Brussels, Belgium, after two explosions rocked the facility on March 22, 2016. A subway station in the city was also targeted in terrorist attacks that killed at least 30 people and injured hundreds more. Investigators say the suspects belonged to the same ISIS network that was behind the Paris terror attacks in November.

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A boy walks past bloodstains and debris at a cafe in Balad, Iraq, that was attacked by ISIS gunmen on May 13, 2016. Twenty people were killed.

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Iraqi government forces patrol in southern Falluja, Iraq, on June 10, 2016. In late June, a senior Iraqi general announced that the battle to reclaim Falluja from ISIS had been won.

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ISIS seized control of Mosul in June 2014 when, according to some accounts, as few as 400 militant fighters routed a force of more than 10,000 soldiers and police, capturing a massive arsenal of U.S.-supplied equipment, including more than 2,000 Humvees. It was a humiliation from which the Iraqi army has never fully recovered.

The Falluja operation has just begun, but the "mission accomplished" moment may be weeks, or even months away.

ISIS has had 2½ years to dig in, make plans, rig improvised explosive devices and other deadly surprises. It took several failed offensives to crush ISIS in the neighboring Ramadi, and much of that city remains uninhabitable.

In Iraq's war against ISIS, nothing is simple. There are no easy victories.